The present invention relates generally to frequency control devices and, in particular, to a surface mounted crystal resonator based oscillator assembly contained in two separate but joined housings.
Frequency control devices are an essential component for many types of mobile communication equipment such as portable telephones, pagers, wireless modems, and personal digital assistants (PDA). To produce a base frequency of oscillation, an electronic oscillator circuit is employed that will typically include a piezoelectric device. Piezoelectric devices can be designed to mechanically resonate at very precise and repeatable frequencies, and this mechanical resonance is translated by the device to an electrical signal. Electrodes are formed upon the surface of the piezoelectric device which enable the application or pick-up of an electric field across some part of the piezoelectric device. In response to electrical stimulation, the piezoelectric device will change physical shape, and when the electrical signal is at the mechanical resonant frequency of the piezoelectric device, energy lost through the piezoelectric device is at a minimum.
Electronic oscillator circuits are typically associated with these piezoelectric devices that set up a basic oscillation, and the piezoelectric device is then used to predicably control the frequency at which the oscillator circuit will be resonant. The mechanical properties of the piezoelectric device are obviously important to the proper functioning of the oscillator, and to accurate frequency control.
Many factors can substantially effect the mechanical properties of a piezoelectric device such as the shape of the piezoelectric material used to make the device, environmental factors including temperature, humidity, and mechanical shock, as well as contamination of the piezoelectric material by minute particles that may contact the surface of the piezoelectric device.
In order to avoid these problems, high quality resonators are generally manufactured in very clean environments such as clean rooms, and are most preferably packaged into a sealed container free of any possible contaminants of corrosive compounds. With regard to the attempts to minimize the effects of temperature, one approach has been to include electrical components or trimming components that compensate for known variations in the frequency of oscillation of the piezoelectric device through a certain temperature range. Crystal oscillators that include such electrical components are referred to as temperature compensated crystal oscillators (TCXO), and are the subject of the present invention.
An example of a TCXO package manufactured for portable wireless applications is shown in U.S. Pat. No. 5,949,294. The TCXO package shown in U.S. Pat. No. 5,949,294 consists of a bottom compartment with a cavity facing upwards, and a top compartment (the crystal resonator assembly), also with a cavity facing upwards. The top compartment is attached to the bottom compartment by metallic connections on the top surface of the bottom compartment and the bottom surface of the top compartment respectively. The top compartment cavity is sealed by a shield plate, and the bottom compartment cavity is sealed by the top compartment.
There are a number of disadvantages with such a stacked TCXO package design. If the cavity of the lower compartment is not potted, the cavity can trap liquids, such as condensed humidity, in the space between the bottom of the cavity and the bottom surface of the top compartment. Another problem with the prior art design is that the size of the TCXO package cannot be minimized substantially without encountering problems. For example, in order to reduce the overall height of the assembled package, attempts have been made to reduce the height of the components placed in the lower compartment, and also by reducing the layer thickness of the sheets that form the bottom compartment. This has resulted in decreased mechanical strength, diminished light blocking, and reduced numbers of possible interconnections between the top and bottom compartments. Another problem with such an arrangement is a lack of design flexibility. Specifically, in this configuration, the crystal resonator assembly must be of sufficient size to seal the bottom compartment.
An alternative design for a TXCO package is shown in U.S. Pat. No. 5,438,219 commonly owned by the present assignee. U.S. Pat. No. 5,438,219 shows a one piece double sided TXCO package having an open top receptacle adapted to receive electronic components and an open bottom receptacle adapted to receive a piezoelectric element and a cover for hermetically sealing the open bottom receptacle. The electronic components and piezoelectric element are electrically connected. A hermetic environment is established by coupling the cover and open bottom receptacle. When viewed in cross section the TXCO package resembles a one piece H-shaped housing.
While the TXCO package of U.S. Pat. No. 5,438,219 alleviates many of the problems with the package described in U.S. Pat. No. 5,949,294, the one piece H-shaped housing creates other problems. Many of these problems comes from the difficulties in manufacturing the one piece H-shaped housing. For example, manufacturing a one piece H-shaped housing involves flipping over the ceramic package that typically makes up the housing. This flipping requires special tooling and necessitates extra handling that could lead to contamination of the piezoelectric element. The flipping also means that one must compensate for sag in the middle of the package cavity similar to the sag in a hammock strung between two posts. Another problem with the one piece H-shaped housing is that there is very little flexibility in creating different size TXCO""s for different applications because the entire housing must be constructed in a single process as opposed to changing just the resonator housing or the oscillator housing. Yet another shortcoming of the one-piece double sided housing design is that if mistakes in assembly are made, entire fabrications must be disposed of.
The present invention provides a packaged resonator and oscillator assembly suitable for use as a double-sided temperature compensated TXCO package. The resonator and the oscillator are in separate but joined housings, and are in electrical communication with one another.
The resonator housing comprises a base having a first or an internal surface, a second or external surface, and sidewalls adjacent to the base of the resonator housing that extend upwardly away from the resonator base. Preferably the sidewalls extend upwardly away from the resonator base in a substantially perpendicular manner. The resonator base and sidewalls form a resonator cavity, which is adapted to receive at least one piezoelectric component, and a cover that is coupled with the resonator sidewalls to form a hermetic enclosure.
The separately manufactured oscillator housing comprises, a base having first and second surfaces, and sidewalls adjacent to the oscillator base extending upwardly away from the oscillator base, preferably in a substantially perpendicular manner. The oscillator base and sidewalls form an oscillator cavity. This oscillator cavity is adapted to receive at least one electronic component, such as a flip chip integrated circuit.
The resonator housing and the oscillator housing are preferably manufactured separately. After the resonator housing and oscillation housing are completed, the external surfaces of the respective bases, i.e., the second surface of the resonator housing base and the second surface of the oscillator housing base, are then joined together such that the piezoelectric component in the resonator housing and the electronic component in the oscillator housing are electrically connected. The electric connection between the resonator housing and the oscillator housing may be accomplished by electrical contacts or lands formed on the external surface of the resonator housing base and the external surface of the oscillator housing base. These electronic contacts may be connected by bonding pads, which may typically be connected by solder, conductive adhesive, ultrasonic or thermosonic welds, a solder ball, or the like. These electrical contacts may alternatively be connected directly by solder, solder ball, conductive adhesive, welding, or the like. Conductive vias extending through the oscillator housing base connect the electronic component in the oscillator housing to the electrical contacts or lands on the oscillator external surface. Conductive vias extending through the resonator housing base also connect the piezoelectric component in the resonator housing with the electrical contacts or lands on the resonator external surface.
When completed, the present invention forms a substantially H-shaped TXCO package in cross-section, depending upon the individual dimensions of the respective housings. This TXCO package avoids the problems of the stacked TXCO discussed above in connection with U.S. Pat. No. 5,949,294. The two piece construction also avoid the manufacturing problems encountered with a one piece H-shaped housing.
The package embodying the present invention permits the resonator housing and the oscillator housing to be fabricated separately, and even in separate locations at different times. Because the resonator housing and the oscillator housing can be manufactured separately, conventional manufacturing equipment may be utilized at a substantial cost saving over specialized manufacturing equipment that is required for a one piece, monolithic H-shaped housing. For example, the oscillator housing may be built using conventional array or panel manufacturing processes. Resonator or crystal units from a multiplicity of sources may also be used and attached to the oscillator section without design changes. The separate assembly also enables greater flexibility in mixing and matching different size resonator housings with different size oscillation housings depending on the particular application and available space. The additional concern about compensating for sag in the middle of the package cavity is also eliminated because both the resonator and oscillator housings can be laid flat on their respective bases during the manufacturing process. Also, there is no need for the additional step of flipping the housings over after either the resonator half or oscillator half of a one piece H-shaped housing is completed.